EYE BRAIN AND VISION - PART 1 pptx

CONTENTS PREFACE 1 INTRODUCTION 2 IMPULSES, SYNAPSES, AND CIRCUITS THE IMPULSE A TYPICAL NEURAL PATHWAY THE VISUAL PATHWAY 3 THE EYE THE EYEBALL THE RETINA THE RECEPTIV

CONTENTS

PREFACE

1 INTRODUCTION

2 IMPULSES, SYNAPSES, AND CIRCUITS

THE IMPULSE

A TYPICAL NEURAL PATHWAY

THE VISUAL PATHWAY

3 THE EYE

THE EYEBALL

THE RETINA

THE RECEPTIVE FIELDS OF RETINAL GANGLION CELLS:

THE OUTPUT OF THE EYE THE CONCEPT OF A RECEPTIVE FIELD

THE OVERLAP OF RECEPTIVE FIELDS

DIMENSIONS OF RECEPTIVE FIELDS

THE PHOTORECEPTORS

BIPOLAR CELLS AND HORIZONTAL CELLS

AMACRINE CELLS

CONNECTIONS BETWEEN BIPOLAR CELLS AND GANGLION CELLS

THE SIGNIFICANCE OF CENTER-SURROUND FIELDS

CONCLUSION

4 THE PRIMARY VISUAL CORTEX

TOPOGRAPHIC REPRESENTATION

RESPONSES OF LATERAL GENICULATE CELLS

LEFT AND RIGHT IN THE VISUAL PATHWAY

LAYERING OF THE LATERAL GENICULATE

RESPONSES OF CELLS IN THE CORTEX

SIMPLE CELLS

COMPLEX CELLS

DIRECTIONAL SELECTIVITY

THE SIGNIFICANCE OF MOVEMENT-SENSITIVE CELLS,

COMMENTS ON HOW WE SEE END STOPPING

THE IMPLICATIONS OF SINGLE-CELL PHYSIOLOGY FOR PERCEPTION

BINOCULAR CONVERGENCE

5 THE ARCHITECTURE OF THE VISUAL CORTEX

ANATOMY OF THE VISUAL CORTEX

LAYERS OF THE VISUAL CORTEX

ARCHITECTURE OF THE CORTEX

EXPLORATION OF THE CORTEX

VARIATIONS IN COMPLEXITY

OCULAR-DOMINANCE COLUMNS

ORIENTATION COLUMNS

MAPS OF THE CORTEX

6 MAGNIFICATION AND MODULES

THE SCATTER AND DRIFT OF RECEPTIVE FIELDS

UNITS OF FUNCTION IN THE CORTEX

DEFORMATION OF THE CORTEX

7 THE CORPUS CALLOSUM AND STEREOPSIS

THE CORPUS CALLOSUM

STUDIES OF THE PHYSIOLOGY OF THE CALLOSUM

STEREOPSIS

THE PHYSIOLOGY OF STEREOPSIS

SOME DIFFICULTIES POSED BY STEREOPSIS

STEREOBLINDNESS

8 COLOR VISION

THE NATURE OF LIGHT

PIGMENTS

VISUAL RECEPTORS

GENERAL COMMENTS ON COLOR

THEORIES OF COLOR VISION

THE GENETICS OF VISUAL PIGMENTS

THE HERING THEORY

COLOR AND THE SPATIAL VARIABLE

THE PHYSIOLOGY OF COLOR VISION: EARLY RESULTS

THE NEURAL BASIS OF COLOR CONSTANCY

BLOBS

CONCLUSION

9 DEPRIVATION AND DEVELOPMENT

RECOVERY

THE NATURE OF THE DEFECT

STRABISMUS

THE ANATOMICAL CONSEQUENCES OF DEPRIVATION

NORMAL DEVELOPMENT OF EYE-DOMINANCE COLUMNS

FURTHER STUDIES IN NEURAL PLASTICITY

THE ROLE OF PATTERNED ACTIVITY IN NEURAL DEVELOPMENT

THE BROADER IMPLICATIONS OF DEPRIVATION RESULTS

10 PRESENT AND FUTURE

FURTHER READING

SOURCES OF ILLUSTRATIONS

INDEX

This book is mainly about the development of our ideas on how the brain handles visual information; it covers roughly the period between 1950 and 1980 The book is

unabashedly concerned largely with research that I have been involved in or have taken a close interest in I count myself lucky to have been around in that era, a time of

excitement and fun Some of the experiments have been arduous, or so it has often

seemed at 4:00 A.M., especially when everything has gone wrong But 98 percent of the time the work is exhilarating There is a special immediacy to neurophysiological

experiments: we can see and hear a cell respond to the stimuli we use and often realize, right at the time, what the responses imply for brain function And in modern science, neurobiology is still an area in which one can work alone or with one colleague, on a budget that is minuscule by the standards of particle physics or astronomy

To have trained and worked on the North American continent has been a special piece of good luck, given the combination of a wonderful university system and a

government that has consistently backed research in biology, especially in vision I can only hope that we have the sense to cherish and preserve such blessings

In writing the book I have had the astronomer in mind as my prototypical reader—

someone with scientific training but not an expert in biology, let alone neurobiology I have tried to give just enough background to make the neurobiology comprehensible, without loading the text down with material of interest only to experts To steer a course between excessive superficiality and excessive detail has not been easy, especially

because the very nature of the brain compels us to look at a wealth of articulated,

interrelated details in order to come away with some sense of what it is and does

All the research described here, in which I have played any part, has been the outcome ofjoint efforts From 1958 to the late 1970s my work was in partner-ship with Torstcn Wiesel Had it not been for his ideas, energy, enthusiasm, stamina, and

willingness to put up with an exasperating colleague, the out-come would have been very different Both of us owe a profound debt to Stephen Kuffler, who in the early years guided our work with the lightest hand imaginable, encouraged us with his boundless enthusiasm, and occasionally discouraged our duller efforts simply by looking puzzled For help in writing one needs critics (I certainly do)—the harsher and more unmerciful, the better I owe a special debt to Eric Kandel for his help with the emphasis in the

opening three chapters, and to my colleague Margaret Livingstone, who literally tore three of the chapters apart One of her comments began, "First you are vague, and then you are snide …" She also tolerated much irascibility and postponement of research To the editors of the Scientific American Library, notably Susan Moran, Linda Davis,

Gerard Piel, and Linda Chaput, and to the copyeditor, Cynthia Farden, I owe a similar debt: I had not realized how much a book depends on able and devoted editors They corrected countless English solecisms, but their help went far beyond that, to spotting duplications, improving clarity, and tolerating my insistence on placing commas and periods after quotation marks Above all, they would not stop bugging me until I had written the ideas in an easily understandable (I hope!) form I want to thank Carol Donner for her artwork, as well as Nancy Field, the designer, Melanie Nielson, the illustration coordinator, and Susan Stetzer, the production coordinator I am also grateful for help in the form of critical reading from Susan Abookire, David Cardozo, Whittemore Tingley,

Deborah Gordon, Richard Masland, and Laura Regan As always, my secretary, Olivia Brum, was helpful to the point of being indispensable and tolerant of my moods beyond any reasonable call of duty My wife, Ruth, contributed much advice and put up with many lost weekends It will be a relief not to have to hear my children say, "Daddy, when are you going to finish that book?" It has, at times, seemed as remote a quest as

SanchoPanza's island

David H Hubel

The changes I have made for this paperback edition consist mainly of minor corrections, the most embarrassing of which is the formula for converting degrees to radians My high school mathematics teachers must be turning over in their graves I have not made any attempt to incorporate recent research on the visual cortex, which in the last ten years has mostly focussed on areas beyond the striate cortex To extend the coverage to include these areas would have required another book I did feel that it would be unforgivable not

to say something about two major advances: the work of Jeremy Nathans on the genetics

of visual pigments, and recent work on the development of the visual system, by Caria Schatz, Michael Stryker, and others

David H Hubel, January 1995

1 INTRODUCTION

Santiago Ramon y Cajal playing chess (white) in 1898, at an age of about 46, while on vacation in Miraflores de

la Sierra This picture was taken by one of his children Most neuroanatomists would agree that Ramon y Cajal stands out far before anyone else in their field and probably in the entire field of central nervous neurobiology His two major contributions were (1) establishing beyond reasonable doubt that nerve cells act as independent units, and (2) using the Golgi method to map large parts 'of the brain and spinal cord, so demonstrating both the extreme complexity and extreme orderliness of the nervous system For his work he, together with Golgi, received the Nobel Prize in 1906

Intuition tells us that the brain is complicated We do complicated things, in immense variety We breathe, cough, sneeze, vomit, mate, swallow, and urinate; we add and subtract, speak, and even argue, write, sing, and compose quartets, poems, novels, and plays; we play baseball and musical instruments We perceive and think How could the organ responsible for doing all that not be complex?

We would expect an organ with such abilities to have a complex structure At the very least, we would expect it to be made up of a large number of elements That alone, however, is not enough to guarantee complexity The brain contains 10ˆ12 (one million million) cells, an astronomical number by any standard I do not know whether anyone has ever counted the cells in a human liver, but I would be surprised if it had fewer cells than our brain Yet no one has ever argued that a liver is as complicated as a brain

We can see better evidence for the brain's complexity in the interconnections between its cells A typical nerve cell in the brain receives information from hundreds or thousands of other nerve cells and in turn transmits information to hundreds or thousands

of other cells The total number of interconnections in the brain should therefore be

somewhere around 10ˆ14 to 10ˆ15, a larger number, to be sure, but still not a reliable index of complexity Anatomical complexity is a matter not just of numbers; more

important is intricacy of organization, something that is hard to quantify One can draw analogies between the brain and a gigantic pipe organ, printing press, telephone

exchange, or large computer, but the usefulness of doing so is mainly in conveying the image of a large number of small parts arranged in precise order, whose functions,

separately or together, the nonexpert does not grasp In fact, such analogies work best if

we happen not to have any idea how printing presses and telephone exchanges work In the end, to get a feeling for what the brain is and how it is organized and handles

information, there is no substitute for examining it, or parts of it, in detail My hope in this book is to convey some flavor of the brain's structure and function by taking a close

The questions that I will be addressing can be simply stated When we look at the outside world, the primary event is that light is focused on an array of 125 million

receptors in the retina of each eye The receptors, called rods and cones, are nerve cells specialized to emit electrical signals when light hits them The task of the rest of the retina and of the brain proper is to make sense of these signals, to extract information that

is biologically useful to us The result is the scene as we perceive it, with all its intricacy

of form, depth, movement, color, and texture We want to know how the brain

accomplishes this feat Before I get your hopes and expectations too high I should warn you that we know only a small part of the answer We do know a lot about the machinery

of the visual system, and we have a fair idea how the brain sets about the task What we know is enough to convince anyone that the brain, though complicated, works in a way that will probably someday be understood—and that the answers will not be so

complicated that they can be understood only by people with degrees in computer science

Today we have a fairly satisfactory understanding of most organs of our body We know reasonably well the functions of our bones, our digestive tubes, our kidneys and liver Not that everything is known about any of these—but at least we have rough ideas: that digestive tubes deal with food, the heart pumps blood, bones support us, and some bones make blood (It would be hard to imagine a time, even in the dark twelfth century, when it was not appreciated that bones are what make our consistency different from that

of an earthworm, but we can easily forget that it took a genius like William Harvey to

discover what the heart does.) What something is for is a question that applies only to

biology, in the broad sense of the word "biology." We can ask meaningfully what a rib is for: it supports the chest and keeps it hollow We can ask what a bridge is for: it lets humans cross a river— and humans, which are part of biology, invented the bridge Purpose has no meaning outside of biology, so that I laugh when my son asks me,

"Daddy, what's snow for?" How purpose comes into biology has to do with evolution, survival, sociobiology, selfish genes—any number of exalted topics that keep many people busy full time Most things in anatomy—to return to solid ground—even such erstwhile mysterious structures as the thymus gland and the spleen, can now have quite reasonable functions assigned to them When I was a medical student, the thymus and spleen were question marks The brain is different Even today large parts of it are

question marks, not only in terms of how they work but also in terms of their biological purpose A huge, rich subject, neuroanatomy consists largely of a sort of geography of

structures, whose functions are still a partial or complete mystery Our ignorance of these regions is of course graded For example, we know a fair amount about the region of brain called the motor cortex and have a rough idea of its function: it subserves voluntary movement; destroy it on one side and the hand and face and leg on the opposite side become clumsy and weak Our knowledge of the motor cortex lies midway along a

continuum of relative knowledge that ranges all the way from utter ignorance of the functions of some brain structures to incisive understanding of a few—like the

understanding we have of the functions of a computer, printing press, internal combustion engine, or anything else we invented ourselves

The visual pathway, in particular the primary visual cortex, or striate cortex, lies near the

bone or heart end of this continuum The visual cortex is perhaps the best-understood part

of the brain today and is certainly the best-known part of the cerebral cortex We know reasonably well what it is "for", which is to say that we know what its nerve cells are doing most of the time in a person's everyday life and roughly what it contributes to the analysis of the visual information This state of knowledge is quite recent, and I can well remember, in the 1950s, looking at a microscopic slide of visual cortex, showing the millions of cells packed like eggs in a crate, and wondering what they all could

conceivably be doing, and whether one would ever be able to find out

This view of a human brain seen from the left and slightly behind shows the cerebral cortex and

cerebellum A small part of the brainstem can be seen just in front of the cerebellum.

How should we set about finding out? Our first thought might be that a detailed

understanding of the connections, from the eye to the brain and within the brain, should

be enough to allow us to deduce how it works Unfortunately, that is only true to a

limited extent The regions of cortex at the back of the human brain were long known to

be important for vision partly because around the turn of the century the eyes were

discovered to make connections, through an intermediate way station, to this part of the

brain But to deduce from the structure alone what the cells in the visual cortex are doing when an animal or person looks at the sky or a tree would require a knowledge of

anatomy far exceeding what we possess even now And we would have trouble even if

we did have a complete circuit diagram, just as we would if we tried to understand a computer or radar set from their circuit diagrams alone—especially if we did not know what the computer or radar set was for

Our increasing knowledge of the working of the visual cortex has come from a

combination of strategies Even in the late 1950s, the physiological method of recording from single cells was starting to tell us roughly what the cells were doing in the daily life

of an animal, at a time when little progress was being made in the detailed wiring

diagram In the past few decades both fields, physiology and anatomy, have gone ahead

in parallel, each borrowing techniques and using new information from the other

I have sometimes heard it said that the nervous system consists of huge numbers of random connections Although its orderliness is indeed not always obvious, I

nevertheless suspect that those who speak of random networks in the nervous system are not constrained by any previous exposure to neuroanatomy Even a glance at a book such

as Cajal's Histologie du Systeme Nerveux should be enough to convince anyone that the

enormous complexity of the nervous system is almost always accompanied by a

compelling degree of orderliness When we look at the orderly arrays of cells in the brain, the impression is the same as when we look at a telephone exchange, a printing press, or the inside of a TV set—that the orderliness surely serves some purpose When confronted with a human invention, we have little doubt that the whole machine and its separate parts have understandable functions To understand them we need only read a set of instructions In biology we develop a similar faith in the functional validity and even ultimately in the understandability of structures that were not invented, but were

perfected through millions of years of evolution The problem of the neurobiologist (to be sure, not the only problem) is to learn how the order and complexity relate to the

function

The principal parts of the nerve cell are the cell body containing the nucleus and other organelles; the single axon, which conveys impulses from the cell; and the dendrites, which receive impulses from other cells

To begin, I want to give you a simplified view of what the nervous system is like— how it is built up, the way it works, and how we go about studying it I will describe typical nerve cells and the structures that are built from them

The main building blocks of the brain are the nerve cells They are not the only cells in the nervous system: a list of all the elements that make up the brain would also include glial cells, which hold it together and probably also help nourish it and remove waste products; blood vessels and the cells that they are made of; various membranes that cover the brain; and I suppose even the skull, which houses and protects it Here I will discuss only the nerve cells

Many people think of nerves as analogous to thin, threadlike wires along which electrical

signals run But the nerve fiber is only one of many parts of the nerve cell, or neuron The

cell body has the usual globular shape we associate with most cells (see diagram on the

next page) and contains a nucleus, mitochondria, and the other organelles that take care

of the many housekeeping functions that cell biologists love to talk about From the cell

body comes the main cylinder-shaped, signal-transmitting nerve fiber, called the axon

Besides the axon, a number of other branching and tapering fibers come off the cell body:

these are called dendrites The entire nerve cell—the cell body, axon, and dendrites—is

The cell body and dendrites receive information from other nerve cells; the axon transmits information from the nerve cell to other nerve cells The axon can be anywhere from less than a millimeter to a meter or more in length; the dendrites are mostly in the millimeter range Near the point where it ends, an axon usually splits into many branches, whose terminal parts come very close to but do not quite touch the cell bodies or

dendrites of other nerve cells At these regions, called synapses, information is conveyed from one nerve cell, the presynaptic cell, to the next, the postsynaptic cell

The signals in a nerve begin at a point on the axon close to where it joins the cell body; they travel along the axon away from the cell body, finally invading the terminal branches At a terminal, the information is transferred across the synapse to the next cell

or cells by a process called chemical transmission, which we take up in Chapter 2 Far

from being all the same, nerve cells come in many different types Although we see some overlap between types, on the whole the distinctiveness is what is impressive No one knows how many types exist in the brain, but it is certainly over one hundred and could

be over one thousand No two nerve cells are identical We can regard two cells of the same class as resembling each other about as closely as two oak or two maple trees do and regard two different classes as differing in much the same way as maples differ from oaks or even from dandelions You should not view classes of cells as rigid divisions: whether you are a splitter or a lumper will determine whether you think of the retina and the cerebral cortex as each containing fifty types of cells or each half a dozen (see the examples on the next page)

Left: The cerebellar Purkinje cell, shown in a drawing by Santiago Ramon y Cajal, presents an extreme in neuronal

specialization The dense dendritic arborization is not bushlike in shape, but is flat, in the plane of the paper, like a cedar frond Through the holelike spaces in this arborization pass millions of tiny axons, which run like telegraph wires perpendicular to the plane of the paper The Purkinje cell's axon gives off a few initial branches close to the cell body and then descends to cell clusters deep in the cerebellum some centimeters away, where it breaks up into nu-

merous terminal branches At life size, the total height of the cell (cell body plus dendrites) is about 1 millimeter

Middle: Ramon y Cajal made this drawing of a pyramidal cell in the cerebral cortex stained The cell body is the small

black blob Right: This drawing by Jennifer Lund shows a cortical cell that would be classed as "stellate" The dark

blob in the center is the cell body Both axons (fine) and dendrites (coarse) branch and extend up and down for distance

of 1 millimeter.

This Golgi stain, in a drawing by Ramon y Cajal, shows a few cells in the upper layers of cerebral cortex in a onc-month-old human baby Only a tiny fraction of a percent of the cells in the area have stained.

The connections within and between cells or groups of cells in the brain are usually not obvious, and it has taken centuries to work out the most prominent pathways Because several bundles of fibers often streak through each other in dense meshworks,

we need special methods to reveal each bundle separately Any piece of brain we choose

to examine can be packed to an incredible degree with cell bodies, dendrites, and axons, with little space between As a result, methods of staining cells that can resolve and reveal the organization of a more loosely packed structure, such as the liver or kidney, produce only a dense black smear in the brain But neuroanatomists have devised

powerful new ways of revealing both the separate cells in a single structure and the connections between different structures.

As you might expect, neurons having similar or related functions are often